ICS calendar subscription is not working with older server using a Letsencrypt certificate

Issue

• Creating new Calendar subscriptions on an iPhone under iOS18 failed
• Multiple (11) existing Calendar subscriptions on the iPhone using the full https URL https://domainname.com/iCal/calendarname.ics were failing after Upgrade to iOS18 with error: "No secure connection possible" with dropping 11 seperate errors in a row, that need to be clicked away when entering the Preferences of the iPhone or Calendar.app. Other sources continued working fine.
• Reentering credentials including url, user, password did not help.
• Existing connections were still syncing new events from the subscription url, with existing credentials, but no changes wer possible.
• Connecting to the same server from Safari using the credential was successful and event were displayed for download or import, but not for subscription

Origin (expected) – from this post about shopify connection issues and [[SSL certificates]] with [[Letsencrypt]] and [[iOS]]

- Problem: On Sept 30th 2021 lets encrypt phased out the old DST Root CA X3 (and it's R3 intermediate) and replaced it with the new ISRG Root X1 Certificate.

- This left tens and millions of websites stack with old certificate(including lots of shopify domain names running on Let's Encrypt SSL's). Older imacs and PC's couldn't update to new certificate hence running into the error above.

Solution - Step by Step (some terms may differ in english because translated back from german UI)

1. On your iPhone download the new current Letsencrypt Root certicate `ISGR Root X1 Certificate` to your Files location on the iPhone from this URL: https://letsencrypt.org/certs/isrgroo
2. Double click downloaded file to install the certificate.
3. Message: Profile loaded Check the Profile in Preferences, if you want to install it [Close].
4. Go to Preferences > `VPN and Devicemanagement`
5. Search for 'X1' certificate and double click it to activate Profile
6. Go To Preferences > General > Info > `Certificate Trust Settings` (`Zertifikatsvertrauenseinstellungen`)
7. Find `When using this certificate` / (`Volles Vertrauen für Root-Zertifikate aktivieren`):
8. Click to select `ISGR Root X1 Certificate`

Now subscribing the ics file should work again!